Action logs are necessary but not sufficient. Knowing that the agent called a tool, sent a message, or returned an output tells you what happened. It doesn't tell you why, and in agentic systems, why is often where the failure lives.

The difference matters most during debugging. An agent produces a wrong output. The action log shows: retrieved document A, called tool B, returned output C. Nothing in that sequence looks wrong — each step was a reasonable action. But the reasoning trace, if you'd captured it, would have shown the agent misinterpreting a sentence in document A in a way that made tool B the logical choice, which made output C the inevitable result. Without the reasoning, you have a mystery. With it, you have a diagnosis.

Reasoning traces also reveal a class of failure that action logs completely miss: the agent that did the right thing for the wrong reason. It retrieved the correct document, but not because it understood the query — because the document happened to contain keywords that matched. It called the right tool, but with parameters that worked by coincidence. These failures are invisible in action logs and visible in reasoning traces, and they matter because the next slightly different input will break the lucky pattern and you won't know why.

The practical objection is cost. Reasoning traces are verbose. Storing them at scale is expensive. This is a real constraint and worth managing — you can sample traces rather than capturing all of them, you can set retention policies that keep recent traces and archive older ones, you can capture full traces only for failed or flagged tasks. These are reasonable tradeoffs. What's not reasonable is capturing nothing and hoping the action log is enough.

There's also a compounding benefit over time. A repository of reasoning traces from real tasks is training material, evaluation data, and institutional knowledge. It's how you understand what your agent actually does versus what you think it does. That understanding is the foundation of every improvement you'll make to the system.

Log the thinking. The actions are just the visible surface of it.